The present invention relates generally to digital communication networks, and more specifically, to networking equipment possessing optical interfaces and conforming to the SONET/SDH standard. The invention provides a system and method for preventing primary section mismatch in Automatic Protection Switching (APS).
SONET/SDH and optical fiber have emerged as significant technologies for building large scale, high speed, Internet Protocol (IP) based networks. SONET and SDH are a set of related standards for synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical Network and SDH is an acronym for Synchronous Digital Hierarchy. A SONET system consists of switches, multiplexers, and repeaters, all connected by fiber. Switches and other components within the SONET/SDH system are configured to have multiple cable connections. For example, nodes are typically connected by at least two lines to provide a backup path.
Automatic Protection Switching is a means to provide SONET/SDH line redundancy. It is described in ITU-T Recommendation G.783 (“Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks”, dated April 1997) and Bellcore GR-253 standard. AnnexB of the ITU-T Recommendation G.783 dated April 1997 describes a multiplex section protection optimized protocol. The protocol provides high availability through SONET line redundancy by switching between two working sections to provide bidirectional non-revertive protection switching. The protocol uses the concept of primary and secondary sections (similar to working and protection lines of Bellcore GR-253 APS) to indicate which working section is carrying traffic. The section that carries traffic when no switch is active is called the primary section (similar to working line) and the other working section (similar to protection line) is called the secondary section. The APS operation switches between the two working sections to realize 1+1 bidirectional non-revertive protection switching. The protocol is exchanged over the K1/K2 bytes in the SONET/SDH frame. Thus, in AnnexB APS, the protection line indication (i.e., primary or secondary) can change between the working sections. This can lead to a situation where APS Line Terminating Equipment (LTE) at different ends indicate different working sections as primary, resulting in primary section mismatch.
According to the Annex B APS protocol, when an APS node moves its primary section from a first working section to a second working section, it expects the remote node to also move to its primary section to the second section. In the event that the two nodes disagree about which section is primary (i.e., one node indicates section one in its APS protocol byte K2 and the other node indicates section two), the node that believed section two was primary changes so that section one is primary and sets its state according to local line conditions and the incoming K2 bytes. The protocol, however, does not specify how much time the side that has switched to the second working section as primary should wait before going back to section one as primary.
With conventional systems, if the remote node does not follow by switching to section two as its primary, and instead continues to indicate that section one is the primary section, the local node continues to wait for the remote node to indicate the new primary section indication in the updated K1/K2 bytes. This may take a long time to happen, or may not happen at all. The same situation may occur after clearing of a user initiated forced switch request. If the remote node is delayed in sending indication about its primary section or is delayed in moving to the other section, the local node which switched to section two will continue to wait for an indication that may or may not arrive, resulting in a primary section mismatch.
For example, if a Signal Fail condition occurs on the primary section, the local and remote selectors both change to working section two as the active line (with primary section still defined as working section one). After the Signal Fail condition clears and a Wait-To-Restore timer fires, the local node changes its K1 byte and sends a No Request on it. The local node will then switch its primary section to working section two. This is reflected in the K2 byte. If the remote node fails to send back a No Request on its K1 byte and continues to indicate working section one as its primary section, the local node will continue to wait for the remote node to switch to working section two, thus resulting in a primary section mismatch.
There is, therefore, a need for a timer based system to resolve primary section mismatch such that a local node switches its primary section indication back to the first working section to resolve a mismatch due to a remote node not switching its primary section to the second working section within a specified period of time.